Mark Wilson October 9th, 2011
Funny word to apply to a fossil: aberrant, meaning “deviating from the normal”. It’s an old-fashioned word rarely used these days, primarily because we have a hard time defining “normal”. It was the word used when I was introduced to the above brachiopod, though, so I employ it in honor of my old-timey professors.
On the left is the dorsal valve exterior and on the right the ventral valve interior of Leptodus americanus Girty 1908. (Both valves are broken.) This species is a member of the Family Lyttoniidae in the Order Productida, which some of my students may have just figured out. The large ventral valve relative to the reduced dorsal valve is the clue. The specimen was found in the Word Limestone (Wordian Stage, Guadalupian Series, Middle Permian System, about 265 million years old) in Hess Canyon, Texas. It is replaced by silica (“silicified”) and so was easily extracted from a block of limestone by dissolving away the calcium carbonate matrix.
These brachiopods, along with many other types, lived in extensive reefs in west Texas during the Permian. The ventral valve was cemented to other shells and extended out parallel to the substrate. The much smaller dorsal valve fit into the grooves, leaving much of the soft-part interior exposed. My professors said it was “like a leaf in a gravy boat” — and I had no idea what a “gravy boat” was then.
It is likely that Leptodus americanus had photosynthetic zooxanthellae embedded in its exposed mantle tissues. These are protists (most often dinoflagellates) that live inside the tissues of metazoans and provide them with nutrients and oxygen in return for carbon dioxide and a cozy place to live. Reef-forming corals are the best known animals to have such a mutualistic symbiotic relationship with zooxanthellae today. It would thus not be surprising to see a similar system with these reefal brachiopods.
Not so aberrant after all.
Girty, G.H. 1908. The Guadalupian fauna. United States Geological Survey Professional Paper 58:1-651.
Williams, A. 1953. The morphology and classification of the oldhaminid brachiopods. Washington Academy of Sciences Journal 9: 279-287.